During a cell search, the user equipment (UE) (e.g., a mobile communication handset) searches for a cell and determines the downlink scrambling code and frame synchronization of that cell. The cell search is typically carried out in three steps: (1) slot synchronization, (2) frame synchronization and code-group identification, and (3) scrambling code identification.
In the slot synchronization phase, the UE uses the SCH's primary synchronization code to acquire slot synchronization to a cell. This is typically done with a single matched filter (or any similar device) matched to the primary synchronization code which is common to all cells. The slot timing of the cell can be obtained by detecting peaks in the matched filter output. In the second step of the cell search procedure, the UE uses the SCH's secondary synchronization code to find frame synchronization and identify the code group of the cell found in the first step. This is done by correlating the received signal with all possible secondary synchronization code sequences, and identifying the maximum correlation value. Since the cyclic shifts of the sequences are unique, the code group as well as the frame synchronization is determined. Lastly, in the final step of the cell search procedure, the UE determines the exact primary scrambling code (PSC) used by the found cell. The primary scrambling code (PSC) is typically identified through symbol-by-symbol correlation over the CPICH with all codes within the code group identified in the second step. After the primary scrambling code (PSC) has been identified, the primary CCPCH can be detected and the system- and cell-specific BCH information can be read.
Once the primary scrambling code (PSC) is identified, subsequent signals received at the UE are identified as coming from the cell via the primary scrambling code (PSC). Consequently, in order for the UE to distinguish between different cells, each cell (BTS) requires a unique primary scrambling code (PSC) out of the 512 codes that are available.